1. Field of the Invention
This invention involves improvements to communications systems and methods in a wireless, frequency division duplex communications system.
2. Description of Related Art
Wireless communications systems, such as cellular and personal communications systems, operate over limited spectral bandwidths. They must make highly efficient use of the scarce bandwidth resource to provide good service to a large population of users. Examples of such communications systems that deal with high user demand and scarce bandwidth resources are wireless communications systems, such as cellular and personal communications systems.
Various techniques have been suggested for such systems to increase bandwidth-efficiency, the amount of information that can be transmitted within a given spectral bandwidth. Many of these techniques involve reusing the same communication resources for multiple users while maintaining the identity of each user""s message. These techniques are generically referred to as multiple access protocols. Among these multiple access protocols are Time Division Multiple Access (TDMA), Code Division Multiple Access (CDMA), Space Division Multiple Access (SDMA), and Frequency Division Multiple Access (FDMA). The technical foundations of these multiple access protocols are discussed in the recent book by Rappaport entitled xe2x80x9cWireless Communications Principles and Practicexe2x80x9d, Prentice Hall, 1996.
The Time Division Multiple Access (TDMA) protocol sends information from a multiplicity of users on one assigned frequency bandwidth by time division multiplexing the information from the various users. In this multiplexing schema particular time slots are devoted to specific users. Knowledge of the time slot during which any specific information is transmitted, permits the separation and reconstruction of each user""s message at the receiving end of the communication channel.
The Code Division Multiple Access (CDMA) protocol uses a unique code to disinguish each user""s data signal from other users"" data signals. Knowledge of the unique code with which any specific information is by permits the separation and reconstruction of each user""s message at the receiving end of the communication channel. There are four types of CDMA protocols classified by modulation: direct sequence (or pseudo-noise), frequency hopping, time hopping, and hybrid systems. The technical foundations for CDMA protocols are discussed in the recent book by Prasad entitled xe2x80x9cCDMA for Wireless Personal Communicationsxe2x80x9d, Artech House, 1996.
The Direct Sequence CDMA (DS-CDMA) protocol spreads a user""s data signal over a wide portion of the frequency spectrum by modulating the data signal with a unique code signal that is of higher bandwidth than the data signal. The frequency of the code signal is chosen to be much larger than the frequency of the data signal. The data signal is directly modulated by the by the code signal and the resulting encoded data signal modulates a single, wideband carrier that continuously covers a wide frequency range. After transmission of the DS-CDMA modulated carrier signal, the receiver uses a locally generated version of the user""s unique code signal to demodulate the received signal and obtain a reconstructed data signal. The receiver is thus able to extract the user""s data signal from a modulated carrier that bears many other users"" data signals.
The Frequency Hopping Spread Spectrum (FHSS) protocol uses a unique code to change a value of the narrowband carrier frequency for successive bursts of the user""s data signal. The value of the carrier frequency varies in time over a wide range of the frequency spectrum in accordance with the unique code. The term Spread Spectrum Multiple Access (SSMA) is also used for CDMA protocols such as DS-CDMA and FHSS that use a relatively wide frequency range over which to distribute a relatively narrowband data signal.
The Time Hopping CDMA (TH-CDMA) protocol uses a single, narrow bandwidth, carrier frequency to send bursts of the user""s data at intervals determined by the user""s unique code. Hybrid CDMA systems include all CDMA systems that employ a combination of two or more CDMA protocols, such as direct sequence/frequency hopping (DS/FH), direct sequence/time hopping (DS/TH), frequency hopping/time hopping (FH/TH), and direct sequence/frequency hopping/time hopping (DS/FH/TH)
The Space Division Multiple Access (SDMA) transmission protocol forms directed beams of energy whose radiation patterns do not overlap spatially with each other, to communicate with users at different locations. Adaptive antenna arrays can be driven in phased patterns to simultaneously steer energy in the direction of selected receivers. With such a transmission technique, the other multiplexing schemes can be reused in each of the separately directed beams. For example, the specific codes used in CDMA can be transmitted in two different beams. Accordingly, if the beams do not overlap each other, different users can be assigned the same code as long as they do not receive the same beam.
The Frequency Division Multiple Access (FDMA) protocol services a multiplicity of users over one frequency band by devoting particular frequency slots to specific users, i.e., by frequency division multiplexing the information associated with different users. Knowledge of the frequency slot in which any specific information resides permits reconstruction of each user""s information at the receiving end of the communication channel.
Orthogonal Frequency Division Multiplexing (OFDM) addresses a problem that is faced, for example, when pulsed signals are transmitted in an FDMA format. In accordance with principles well known in the communication sciences, the limited time duration of such signals inherently broadens the bandwidth of the signal in frequency space. Accordingly, different frequency channels may significantly overlap, defeating the use of frequency as a user-identifying-parameter, the principle upon which FDMA is based. However, pulsed information that is transmitted on specific frequencies can be separated, in accordance with OFDM principles, despite the fact that the frequency channels overlap due to the limited time duration of the signals. OFDM requires a specific relationship between the data rate and the carrier frequencies. Specifically, the total signal frequency band is divided into N frequency sub-channels, each of which has the same data rate 1/T. These data streams are then multiplexed onto a multiplicity of carriers that are separated in frequency by 1/T. Multiplexing signals under these constraints results in each carrier having a frequency response that has zeroes at multiples of 1/T. Therefore, there is no interference between the various carrier channels, despite the fact that the channels overlap each other because of the broadening associated with the data rate. OFDM is disclosed, for example, by Chang in Bell Sys. Tech Jour., Vol. 45, pp. 1775-1796, December 1966, and in U.S. Pat. No. 4,488,445.
Parallel Data Transmission is a technique related to FDMA. It is also referred to as Multitone Transmission (MT), Discrete Multitone Transmission (DMT) or Multi-Carrier Transmission (MCT). Parallel Data Transmission has significant calculational advantages over simple FDMA. In this technique, each user""s information is divided and transmitted over different frequencies, or xe2x80x9ctonesxe2x80x9d, rather than over a single frequency, as in standard FDMA. In an example of this technique, input data at NF bits per second are grouped into blocks of N bits at a data rate of F bits per second. N carriers or xe2x80x9ctonesxe2x80x9d are then used to transmit these bits, each carrier transmitting F bits per second. The carriers can be spaced in accordance with the principles of OFDM.
Both the phase and the amplitude of the carrier can be varied to represent the signal in multitone transmission. Accordingly, multitone transmission can be implemented with M-ary digital modulation schemes. In an M-ary modulation scheme, two or more bits are grouped together to form symbols and one of the M possible signals is transmitted during each symbol period. Examples of M-ary digital modulation schemes include Phase Shift Keying (PSK), Frequency Shift Keying (FSK), and higher order Quadrature Amplitude Modulation (QAM). In QAM a signal is represented by the phase and amplitude of a carrier wave. In high order QAM, a multitude of points can be distinguished on a amplitude/phase plot. For example, in 64-ary QAM, 64 such points can be distinguished. Since six bits of zeros and ones can take on 64 different combinations, a six-bit sequence of data symbols can, for example, be modulated onto a carrier in 64-ary QAM by transmitting only one value set of phase and amplitude, out of the possible 64 such sets.
Suggestions have been made to combine some of the above temporal and spectral multiplexing techniques. For example, in U.S. Pat. No. 5,260,967, issued to Schilling, there is disclosed the combination of TDMA and CDMA. In U.S. Pat. No. 5,291,475, issued to Bruckert, and in U.S. Pat. No. 5,319,634 issued to Bartholomew, the combination of TDMA, FDMA, and CDMA is suggested.
Other suggestions have been made to combine various temporal and spectral multiple-access techniques with spatial multiple-access techniques. For example, in U.S. Pat. No. 5,515,378, filed Dec. 12, 1991, Roy suggests xe2x80x9cseparating multiple messages in the same frequency, code, or time channel using the fact that they are in different spatial channels.xe2x80x9d Roy suggests specific application of his technique to mobile cellular communications using an xe2x80x9cantenna arrayxe2x80x9d. Similar suggestions were made by Swales et. al., in the IEEE Trans. Veh. Technol. Vol. 39. No. 1 February 1990, and by Davies et. al. in A.T.R., Vol. 22, No. 1, 1988 and in Telecom Australia, Rev. Act., 1985/86 pp. 41-43.
Gardner and Schell suggest the use of communications channels that are xe2x80x9cspectrally disjointxe2x80x9d in conjunction with xe2x80x9cspatially separablexe2x80x9d radiation patterns in U.S. Pat. No. 5,260,968, filed Jun. 23, 1992. The radiation patterns are determined by restoring xe2x80x9cself coherencexe2x80x9d properties of the signal using an adaptive antenna array. xe2x80x9c[A]n adaptive antenna array at a base station is used in conjunction with signal processing through self coherence restoral to separate the temporally and spectrally overlapping signals of users that arrive from different specific locations.xe2x80x9d See the Abstact of the Invention. In this patent, however, adaptive analysis and self coherence restoral is only used to determine the optimal beam pattern; xe2x80x9c . . . conventional spectral filters . . . [are used] . . . to separate spatially inseparable filters.xe2x80x9d
Winters suggests xe2x80x9cadaptive array processingxe2x80x9d in which xe2x80x9c[t]he frequency domain data from a plurality of antennas are . . . combined for channel separation and conversion to the time domain for demodulation,xe2x80x9d in U.S. Pat. No. 5,481,570, filed Oct. 20, 1993. Column 1 lines 66-67 and Column 2, lines 14-16.
Agee has shown that xe2x80x9cthe use of an M-element multiport antenna array at the base station of any communication network can increase the frequency reuse of the network by a factor of M and greatly broaden the range of input SINRs required for adequate demodulation . . . xe2x80x9d (xe2x80x9cWireless Personal Communications: Trends and Challengesxe2x80x9d, Rappaport, Woerner and Reed, editors, Kluwer Academic Publishers, 1994, pp. 69-80, at page 69. see also, Proc. Virginia Tech. Third Symposium on Wireless Personal Communications, June 1993, pp. 15-1 to 15-12.).
Gardner and Schell also suggest in U.S. Pat. No. 5,260,968, filed Jun. 23, 1992, xe2x80x9ctime division multiplexing of the signal from the base station and the usersxe2x80x9d . . . xe2x80x9c[i]n order to use the same frequency for duplex communications . . . xe2x80x9d xe2x80x9c[R]ecption at the base station from all mobile units is temporally separated from transmission from the base station to all mobile units.xe2x80x9d Column 5, lines 44ff In a similar vein, in U.S. Pat. No. 4,383,332 there is disclosed a wireless multi-element adaptive antenna array SDMA system where all the required adaptive signal processing is performed at baseband at the base station through the use of xe2x80x9ctime division retransmission techniques.xe2x80x9d
Fazel, xe2x80x9cNarrow-Band Interference Rejection in Orthogonal Multi-Carrier Spread-Spectrum Communicationsxe2x80x9d, Record, 1994 Third Annual International Conference on Universal Personal Communications, IEEE, 1994, pp. 46-50 describes a transmission scheme based on combined spread spectrum and OFDM. A plurality of subcarrier frequencies have components of the spreaded vector assigned to them to provide frequency-diversity at the receiver site. The scheme uses frequency domain analysis to estimate interference, which is used for weighting each received subcarrier before despreading. This results in switching off those subcarriers containing the interference.
Despite the suggestions in the prior art to combine certain of the multiple access protocols to improve bandwidth efficiency, there has been little success in implementing such combinations. It becomes more difficult to calculate optimum operating parameters as more protocols are combined. The networks implementing combined multiple access protocols become more complex and expensive. Accordingly, the implementation of high-bandwidth efficiency communications using a combination of multiple access protocols continues to be a challenge.
The invention enables high quality PCS communications in environments where adjacent PCS service bands operate with out-of-band harmonics that would otherwise interfere with the system""s operation. The highly bandwidth-efficient communications method combines a form of time division duplex (TDD), frequency division duplex (FDD), time division multiple access (TDMA), orthogonal frequency division multiplexing (OFDM), spatial diversity, and polarization diversity in various unique combinations. The invention provides excellent fade resistance. The invention enables changing a user""s available bandwidth on demand by assigning additional TDMA slots during the user""s session.
In one embodiment of the invention, TDD, FDD, TDMA, and OFDM are combined to enable a base station to efficiently communicate with many remote stations. The method includes the step of receiving at the base station a first incoming wireless signal comprising a plurality of first discrete frequency tones that are orthogonal frequency division multiplexed (OFDM) in a first frequency band from a first remote station during a first time division multiple access (TDMA) interval. Then the method includes the step of receiving at the base station a second incoming wireless signal comprising a plurality of second discrete frequency tones that are orthogonal frequency division multiplexed (OFDM) in the first frequency band from a second remote station during the first time division multiple access (TDMA) interval. The first and second stations accordingly have different sets of discrete frequency tones that are orthogonal frequency division multiplexed.
Then the method includes the step of receiving at the base station a third incoming wireless signal comprising a plurality of the first discrete frequency tones that are orthogonal frequency division multiplexed (OFDM) in the first frequency band from a third remote station during a second time division multiple access (TDMA) interval. The first and third stations accordingly are time division multiplexed by sharing the same set of discrete frequency tones in different TDMA intervals.
Then the method includes the step of receiving at the base station a fourth incoming wireless signal comprising a plurality of the second discrete frequency tones that are orthogonal frequency division multiplexed (OFDM) in the first frequency band from a fourth remote station during the second time division multiple access (TDMA) interval. The second and fourth stations accordingly are time division multiplexed by sharing the same set of discrete frequency tones in different TDMA intervals.
Then the method includes the step of transmitting at the base station the first outgoing wireless signal comprising a plurality of third discrete frequency tones that are orthogonal frequency division multiplexed (OFDM) in a second frequency band to the first remote station during a third time division multiple access (TDMA) interval. The first remote station and the base station accordingly are time division duplexed (TDD) by transmitting their respective signals at different TDMA intervals. In addition, the first remote station and the base station accordingly are frequency division duplexed (FDD) by transmitting their respective signals on different sets of discrete frequency tones in different frequency bands.
Then the method includes the step of transmitting at the base station the second outgoing wireless signal comprising a plurality of fourth discrete frequency tones that are orthogonal frequency division multiplexed (OFDM) in the second frequency band to the second remote station during the third time division multiple access (TDMA) interval. The second remote station and the base station accordingly are time division duplexed (TDD) by transmitting their respective signals at different TDMA intervals. In addition, the second remote station and the base station accordingly are frequency division duplexed (FDD) by transmitting their respective signals on different sets of discrete frequency tones in different frequency bands.
Then the method includes the step of transmitting at the base station the third outgoing wireless signal comprising the plurality of the third discrete frequency tones that are orthogonal frequency division multiplexed (OFDM) in the second frequency band to the third remote station during a fourth time division multiple access (TDMA) interval. The third remote station and the base station accordingly are time division duplexed (TDD) by transmitting their respective signals at different TDMA intervals. In addition, the third remote station and the base station accordingly are frequency division duplexed (FDD) by transmitting their respective signals on different sets of discrete frequency tones in different frequency bands.
Then the method includes the step of transmitting at the base station the fourth outgoing wireless signal comprising the plurality of the fourth discrete frequency tones that are orthogonal frequency division multiplexed (OFDM) in the second frequency band to the fourth remote station during the fourth time division multiple access (TDMA) interval. The fourth remote station and the base station accordingly are time division duplexed (TDD) by transmitting their respective signals at different TDMA intervals. In addition, the fourth remote station and the base station accordingly are frequency division duplexed (FDD) by transmitting their respective signals on different sets of discrete frequency tones in different frequency bands.
In another embodiment of the invention, TDD, FDD, TDMA, OFDM, and space diversity are combined to enable a base station to efficiently communicate with many remote stations. This is possible because of the multiple element antenna array at the base station that is controlled by despreading and spreading weights. The spreading weights enable the base station to steer the signals it transmits to remote stations that are have a sufficient geographic separation from one another. The despreading weights enable the base station to steer the receive sensitivity of the base station toward the sources of signals transmits by remote stations that have a sufficient geographic separation from one another.
The method includes the step of receiving at the base station a first incoming wireless signal comprising a plurality of first discrete frequency tones that are orthogonal frequency division multiplexed (OFDM) in a first frequency band from a first remote station at a first geographic location during a first time division multiple access (TDMA) interval. Then the method includes the step of receiving at the base station a second incoming wireless signal comprising a plurality of the first discrete frequency tones that are orthogonal frequency division multiplexed (OFDM) in the first frequency band from a second remote station at a second geographic location during the first time division multiple access (TDMA) interval. Then the method includes the step of spatially despreading the first and second incoming signals received at the base station by using spatial despreading weights. Spatial diversity is provided because the despreading weights enable the base station to steer the receive sensitivity of the base station toward the first remote station and the second remote station, respectively.
Later, the method performs the step of spatially spreading a first and second outgoing wireless signals at the base station by using spatial spreading weights. Then the method includes the step of transmitting at the base station the first outgoing wireless signal comprising a plurality of third discrete frequency tones that are orthogonal frequency division multiplexed (OFDM) in a second frequency band to the first remote station at the first geographic location during a third time division multiple access (TDMA) interval. Then the method includes the step of transmitting at the base station the second outgoing wireless signal comprising a plurality of the third discrete frequency tones that are orthogonal frequency division multiplexed (OFDM) in the second frequency band to the second remote station at the second geographic location during the third time division multiple access (TDMA) interval. Spatial diversity is provided because the spreading weights enable the base station to steer the signals it transmits to the first and second remote stations, respectively.
In another embodiment of the invention, TDD, FDD, TDMA, OFDM, and polarization diversity are combined to enable a base station to efficiently communicate with many remote stations. This is possible because the antenna at the base station and the antennas at the remote stations are designed to distinguish orthogonally polarized signals. Signals exchanged between the base station and a first remote station are polarized in one direction, and signals exchanged between the base station and a second remote station are polarized in an orthogonal direction.
The method includes the step of receiving at the base station a first incoming wireless signal polarized in a first polarization direction comprising a plurality of first discrete frequency tones that are orthogonal frequency division multiplexed (OFDM) in a first frequency band from a first remote station during a first time division multiple access (TDMA) interval. Then the method includes the step of receiving at the base station a second incoming wireless signal polarized in a second polarization direction comprising a plurality of the first discrete frequency tones that are orthogonal frequency division multiplexed (OFDM) in the first frequency band from a second remote station during the first time division multiple access (TDMA) interval. Then the method includes the step of distinguishing the first and second incoming signals received at the base station by detecting the first and second polarization directions. Polarization diversity is provided because signals exchanged between the base station and the first remote station are polarized in one direction, and signals exchanged between the base station and the second remote station are polarized in an orthogonal direction.
Later, the method includes the step of forming a first and second outgoing wireless signals at the base station by polarizing them in the first and second polarization directions, respectively. Then the method includes the step of transmitting at the base station the first outgoing wireless signal polarized in the first polarization direction comprising a plurality of third discrete frequency tones that are orthogonal frequency division multiplexed (OFDM) in a second frequency band to the first remote station at the first geographic location during a third time division multiple access (TDMA) interval. Then the method includes the step of transmitting at the base station the second outgoing wireless signal polarized in the second polarization direction comprising a plurality of the third discrete frequency tones that are orthogonal frequency division multiplexed (OFDM) in the second frequency band to the second remote station at the second geographic location during the third time division multiple access (TDMA) interval. Polarization diversity is provided because signals exchanged between the base station and the first remote station are polarized in one direction, and signals exchanged between the base station and the second remote station are polarized in an orthogonal direction.
In still another embodiment of the invention, TDD, FDD, TDMA, OFDM, spatial diversity, and polarization diversity are combined to enable a base station to efficiently communicate with many remote stations. The resulting invention makes highly efficient use of scarce bandwidth resources to provide good service to a large population of users.
Currently, the invention has advantageous applications in the field of wireless communications, such as cellular communications or personal communications, where bandwidth is scarce compared to the number of the users and their needs. Such applications may be effected in mobile, fixed, or minimally mobile systems. However, the invention may be advantageously applied to other, non-wireless, communications systems as well.